T cell homeostasis: Keeping useful T cells alive and live T cells useful

It has become clear that the regulation of T cell numbers is under peripheral homeostatic control. However, the rules for homeostasis vary with the T cells' differentiation state and the overall T cell number in the animal. Furthermore, homeostatic pressures can cause unexpected changes in T cell differentiation and function which might promote or dampen T cell reactivity. In this review, we focus on the role of peptide/MHC and cytokine interactions in regulating the size and composition of the T cell pool.